In Geology, a rock is a naturally occurring aggregate of one or more minerals or mineraloids generally found in a solid state. For example, the common rock granite is a combination of the quartz, feldspar and biotite minerals. The outermost solid layer of the Earth, the Lithosphere, is made of rock.
Throughout history rocks have been used by the mankind. From the Stone Age rocks have been used as tools for hunting and agriculture. The minerals and metals we find in rocks have been essential to human civilization. About 87 percent of minerals in earth’s crust are silicates.
The scientific study of rocks is called Petrology.
Rocks are classified into three basic groups based on how they were formed in nature. Rocks are usually classified into three major types-
1. Igneous rocks,
2. Sedimentary rocks,and
3. Metamorphic rocks.
Types of Rocks
1. Igneous rocks – Important facts related to Igneous rocks are given below:-
(a) Igneous rocks forms through the cooling and solidification of magma (molten rock deep inside the earth) or lava. When rocks are buried deep within the earth, they melt because of the high pressure and temperature inside the earth. It erupts onto the surface from volcanoes, and cools and solidify to take the form of Igneous rocks.
(b) Igneous rocks made up 85 percent or more of the earth’s crust.
(c) Igneous rocks are divided into two main categories: plutonic rock and volcanic rock.
Plutonic rock – Plutonic or intrusive rocks are formed when magma cools and crystallizes slowly within the Earth’s crust.
Example – Granite
Volcanic rock – Volcanic or extrusive rock are formed by the cooling and solidification of molten lava on the surface of the earth.
Example – Basalt.
2. Sedimentary rocks – Important facts related to Sedimentary rocks are given below:-
(a) Sedimentary rocks are formed by the deposition of organic and non-organic matter over a period of time. The process of deposition is called sedimentation.
(b) Sedimentary rocks often contain fossils.
(c) About 75 percent surface area of the earth is covered by the sedimentary rocks.
(d) Sedimentary rocks hold all reserves of coal, oil and natural gas.
(e) Example – Lignite, Coal, Sandstone, Shale.
3. Metamorphic rocks – Important facts related to Igneous rocks are given below:-
(a) In Greek, the word metamorphic means ‘change of form’.
(b) Due to unfavourable condition of pressure and change in temperature under the earth’s surface Igneous and Sedimentary rocks undergo change. This change leads to the formation of Metamorphic Rocks.
(c) This process of change of form is called Metamorphism. Physcial and chemical properties of the stone change drastically. Shale may transformed into slate, limestone into marble, sandstone into quartzite, etc.
(d) Metamorphic rocks may be classified as foliated or non-foliated.
Foliated metamorphic rocks appeared bended or layered. Example – Mica. Non-foliated metamorphic rock usually contains one mineral. It is uniform in texture. Example – Quartzite and marble.